The present invention is generally directed to methods for providing coatings on selected portions of a substrate. In more particular embodiments, the invention relates to masking techniques used in the application of protective coatings to metallic substrates employed in high temperature applications.
Metal alloys are often used in industrial environments which include extreme operating conditions. For example, the alloys may be exposed to high temperatures, e.g., above about 750° C. Moreover, the alloys may be subjected to repeated temperature cycling, e.g., exposure to high temperatures, followed by cooling to room temperature, and then followed by rapid re-heating. As an example, gas turbine engines are often subjected to repeated thermal cycling during operation.
The turbine engine components (and other industrial parts) are often formed of superalloys, which are usually nickel-, cobalt-, or iron-based. Superalloys can withstand a variety of extreme operating conditions. However, they often must be covered with coatings which protect them from environmental degradation, e.g., the adverse effects of corrosion and oxidation.
Various types of coatings are used to protect superalloys and other types of high-performance metals. As one example, a thermal barrier coating (TBC) system is often employed. The TBC system includes a bond layer (usually an MCrAlX material, as described below) and a ceramic overcoat, such as a zirconia-based material. The bond layer promotes adhesion between the substrate and the ceramic overcoat. Many techniques are available to apply the TBC systems. Examples include vacuum plasma deposition (VPS) and air plasma spray (APS).
It is usually very important that protective coatings like the TBC's are deposited on very specific areas of a substrate. As an example, various sections of a turbine engine nozzle need to be completely covered with a highly adherent TBC system, since these sections are exposed to high-temperature combustion gasses. However, in many instances, adjacent sections of the nozzle must be completely free of the protective coatings. For example, some of the “slash faces” of a nozzle may have to be eventually welded. Therefore, they must not contain any coating material which would interfere with the welding process. A “clean”, distinct edge between a coated and an uncoated section of a substrate is often critical to the overall coating process.
To provide one specific example, FIG. 1 is a perspective of a portion of a typical turbine airfoil 10, which includes superalloy substrate 12. In some instances, it is necessary to apply a high-quality TBC system on the surface of substrate 12, as well as on platform 14 and vertical face 16. However, it may also be necessary to keep surface 18 entirely free of TBC or bond coat material. In other words, corner-edge 20 must sometimes be, ultimately, very clean. Top platform 22 is depicted in simplified form, and would also usually include various surfaces which are to be coated, along with adjacent surfaces which are to remain uncoated.
Various techniques are available for ensuring that a clean edge is formed between a coated and uncoated section of a substrate. Usually, the surfaces which are to remain uncoated are taped off prior to the coating deposition step. Often, masks are also used—alone or in conjunction with tape—to prevent deposition of a coating onto an area covered by the pattern of the mask. Mask or tape may also be used to protect sections of the substrate from various pre-treatment steps. As an example, a grit-blasting step is often necessary to roughen a surface prior to the thermal spray-deposition of a coating. It is usually necessary to tape off other sections which are not to be coated, since grit blasting can damage those sections. After the desired coating is deposited, the mask and tape can be removed. Additional coating steps or other treatment steps can then be carried out, which may involve additional masking and taping steps.
While masking techniques are effective in many coating processes, they sometimes exhibit serious drawbacks. “Hard” metal masks have typically been used in the past. They are usually very thick (at least about 0.125 inch (3.2 mm)), and made of high temperature materials like nickel-based superalloys. It is often time-consuming to position these hard masks on specific sections of the substrate. Moreover, the hard masks are not pliable, and do not easily conform to sections of the substrate which are curved or contain many indentations. Since that conformance is often necessary, hard masks may have to be pre-shaped in separate metalworking procedures, which can be costly and time-consuming.
Furthermore, the “clean edge” discussed above is often not possible when a hard mask is used. Such a mask is usually positioned at the boundary between a substrate section to be coated and another section which is to be left uncoated. As coating material is deposited, a hump or ridge of material is built up at the edge adjacent the uncoated section. (This occurrence is often referred to as “bridging”). When the mask is removed, chipping and cracking often occurs along the edge. This problem can be especially severe in the case of ceramic overcoats (e.g., zirconia-based coatings), which have high cohesive strength in a direction horizontal to the plane of the substrate. The force needed to remove the hard mask can cause a significant amount of the desired coating to be pulled off the coated section of the substrate.
In the case of turbine engine components, the loss of any coating material can be a very serious problem, since the edge of the underlying part is then directly exposed to very high in-service temperatures. Moreover, chipping and cracking along the edge can serve as crack propagation sites for further degradation throughout the coating. Thus, time-consuming steps have to be undertaken to repair the coating imperfections. More often, the parts must be entirely stripped and re-coated. These steps add considerable expense to the overall manufacturing process.
Other masking procedures can be used in place of the hard masks. For example, chemical masking techniques are sometimes used, in which a non-bonding coating (e.g., an “anti-bond” material) is sprayed or painted on selected portions of the substrate. These types of non-bonding materials prevent the protective coatings from permanently adhering to the substrate.
While chemical masking techniques are effective in some applications, they often include drawbacks as well. For example, it often takes about 8-16 hours for the chemical mask to dry, prior to protective coating deposition. Moreover, additional steps are needed to remove the chemical mask after coating deposition is complete. A variety of solvents and additive-based solutions may have to be used to completely remove the chemical mask, since any chemical residue may interfere with subsequent coating, treatment, or welding operations.
It should thus be apparent that new masking processes would be very welcome in the field of protective coating technology. Use of the processes should result in a very clean edge or boundary between coated and uncoated sections of a substrate. The processes should be capable of being carried out in an efficient manner, without causing delays in any other steps undertaken in processing the substrate. Moreover, the processes should be compatible with techniques used to apply protective ceramic and metallic coatings to superalloy substrates—including those which may be curved and/or which may contain various cavities or indentations.